In recent years, users can easily obtain contents including multi-channel audio (surround sound) via broadcast waves, disk media such as Digital Versatile Discs (DVDs) and Blu-Ray (trade name) Discs (BD), the Internet, and the like. In movie theaters and the like, a large number of stereophonic systems using object-based audio, such as Dolby Atmos, are deployed. Furthermore, in Japan, as 22.2 ch audio is adopted as the next generation broadcast standard, for example, opportunities for users to experience multi-channel contents have dramatically increased.
Various investigations of multi-channel conversion methods for known stereophonic audio signals have also been conducted, and a technique for multi-channel conversion based on the correlation between the channels of a stereo signal is disclosed in PTL 2, for example. Also, with respect to systems that play multi-channel audio, in addition to facilities equipped with large-scale audio equipment such as movie theaters and halls, systems that can be enjoyed easily at home and the like are becoming more common. By placing a plurality of speakers according to arrangement standards recommended by the International Telecommunication Union (ITU) (see NPL 1), a user (listener) can construct an environment for listening to multi-channel audio such as 5.1 ch or 7.1 ch in a home. In addition, techniques have been studied that reproduce multi-channel stereo image localization with a small number of speakers (NPL 2).